can anyone explain this???I freed all slaves in the city and that made it go to zero, but why was it minus in the first place???

Because when you took over the city, the slaves were taken out of the city as "loot" the amount of slaves you had will have gone up by 794 (number above the clock). This happened to me yesterday however as I always keep slaves in a "safe" city I could see that my slave numbers had increased.
I think this is a random thing as some cities I have taken the slaves remain in the city. DoD